As a fixing device used for an image forming apparatus such as an electrophotographic copier or printer, there is a fixing device in which a sheet paper is inserted into a nip formed between a pair of rollers including a heat roller and a pressure roller, or between similar fixing belts, and a toner image is heated, pressurized and fixed. In the thermal fixing device as stated above, in order to keep the heat roller or the fixing belt at a constant fixable temperature, the surface temperature of the heat roller or the fixing belt is detected by a temperature sensor, and a heat source is ON/OFF controlled according to the detection result.
As the temperature sensor, in recent years, like an infrared temperature sensor, there is used a non-contact temperature sensor to perform temperature detection without contact with a heating target member, such as a heat roller or a fixing belt. Especially, a thermopile infrared temperature sensor has a structure in which the heat capacity of a hot junction part of a thin film thermocouple is made small, and the temperature response is high. As a result, it becomes possible to quickly perform the temperature control of the heating target member with high precision. As the thermopile infrared temperature sensor, there are a monocular infrared temperature sensor to detect reflected infrared rays from one place, and a multiple infrared temperature sensor to detect reflected infrared rays from plural positions at the same time.
However, the non-contact temperature sensor as stated above detects also the temperature of a contamination on a surface of an object for which the temperature detection is performed, and a contamination on the surface of the non-contact temperature sensor. Thus, there is a fear that the accurate surface temperature of the object can not be obtained, and erroneous detection occurs. Further, there is a fear that dust or dirt, such as scattered toner or paper powder, is attached to the surface of the heating target member in the fixing device or the non-contact temperature sensor. Further, while the fixing operation is performed, the detection precision of the non-contact temperature sensor is reduced, and there is a fear that an error occurs in the detection result.
Thus, in the non-contact temperature sensor, as a device to prevent the occurrence of temperature error due to the environmental change or contamination, for example, JP-A-2005-24436, JP-A-2000-259034 and JP-A-2005-259033 disclose devices to correct temperature error of a non-contact temperature sensor using a detection result of a contact temperature sensor. In these devices, at the time of a correction operation, the non-contact temperature sensor is mechanically rotated so as to detect the temperature of the same position as the detection position of the contact temperature sensor.
However, any of these non-contact temperature sensors of the related art are monocular infrared temperature sensors, and one place of the fixing device is temperature-detected. On the other hand, in order to temperature-control the fixing device with higher precision, it is preferable to perform the temperature detection of plural positions of the fixing device. However, in the non-contact temperature sensor of the related art, in order to detect temperatures of plural positions of the fixing device, while the fixing is performed, the non-contact temperature sensor must be rotated at high speed, and it is not suitable for practical application.
Then, in a fixing device to detect the surface temperature of a heating target member by using a non-contact temperature sensor, it is desired to develop a fixing device in which temperatures of plural positions of the heating target member are detected in a non-contact manner without rotating the non-contact temperature sensor, the contamination of the surface of an object or the non-contact temperature sensor, and reduction in the detection precision of the non-contact temperature sensor are quickly dealt with, temperature detection of the heating target member is performed in a short time and with high precision, fixing properties are improved, and high picture quality is obtained.